System for controlling a transfer clutch of a four-wheel drive vehicle

ABSTRACT

A four-wheel drive vehicle has an engine clutch for transmitting the output of an engine to a transmission, main drive wheels operatively connected to an output shaft of the transmission, auxiliary drive wheels, and a transfer clutch for transmitting the output of the transmission to the auxiliary drive wheels. A control system is provided with a detector for detecting a driving condition for reducing the torque capacity of the transfer clutch. The control system responds to a signal from the detector for gradually reducing the torque capacity of the transfer clutch.

BACKGROUND OF THE INVENTION

The present invention relates to a control system for four-wheel drivevehicles, and more particularly to a system for controlling the torquecapacity of a transfer clutch for four-wheel drive vehicles.

A four-wheel drive vehicle of the type in which a transfer clutch isprovided for transmitting the output torque of a transmission toauxiliary drive wheels is known. In such a vehicle, a power transmissionsystem can be converted from a two-wheel drive power transmission trainto a four-wheel drive Cpower transmission train by engaging the transferclutch. Meanwhile, when the vehicle negotiates corners, brakingphenomenon called "tight corner braking" will occur at a sharp corner.Japanese Patent Application Laid Open No. 57-84227 discloses a systemwhich is intended to eliminate the braking phenomenon. The disclosedsystem is provided with a fluid operated friction clutch as a transferclutch, and a four-wheel drive power transmission system is converted toa two-wheel drive power transmission system by disengaging the clutchwhen a vehicle turns a sharp corner. However, when oil is dischargedfrom the clutch, pressure of oil in the clutch decreases quickly.Accordingly, the torque transmitted by the clutch changes quickly andlargely, which causes a great shock in the vehicle at the change of thepower transmission train.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a control system whichgradually varies the clutch pressure, thereby reducing the shock whichoccurring at the change of the power transmission train.

The other objects and features of this invention will become apparentlyunderstood from the following description with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1a to 1c show a sectional view of an infinitely variablebelt-drive transmission to which the present invention is applied;

FIGS. 2a to 2c show a hydraulic control circuit according to the presentinvention;

FIGS. 3a to 3c show a block diagram of a control system of the presentinvention;

FIG. 4a is a graph showing a relationship between clutch pressure,control pressure and duty ratio;

FIG. 4b shows a characteristic of an electromagnetic valve;

FIG. 4c shows a relationship between vehicle speed and engine speed;

FIGS. 5a to 5c show the a flowchart showing main operation of thesystem;

FIG. 6 a relationship between engine speed and torque capacity of atransfer clutch in a low vehicle speed range; and

FIG. 7 shows characteristics of a transfer clutch in the system of thepresent invention compared to prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1a to 1c show a four-wheel drive power transmission system with aninfinitely variable belt-drive automatic transmission for a motorvehicle, to which the present invention is applied. An engine E istransversely mounted on a motor vehicle at a front portion thereof. Thepower transmission system comprises an electromagnetic powder clutch 1,a selector device 2, an infinitely variable belt-drive transmission 3, adifferential 4 for front wheels, and a transfer device 5. Theelectromagnetic powder clutch 1 is provided in a housing 6. The selectordevice 2, transmission 3, final differential 4 and transfer device 5 areprovided in a main housing 7 and a side housing 8. An extension case 9is secured to the main housing 7. A crankshaft 10 of the engine E isconnected to an annular drive member 12 through a drive plate 11 of theelectromagnetic powder clutch 1. The electromagnetic powder clutch 1comprises a driven member 14, and a magnetizing coil 15 provided in thedriven member 14. The driven member 14 has its outer periphery spacedfrom the inner periphery of the drive member 12 by a gap 16. Magneticpowder material is provided in the gap 16. The driven member 14 issecured to an input shaft 13 of the belt-drive transmission. A holdersecured to the driven member 14 carries slip rings 18 which areelectrically connected to the coil 15. The coil 15 is supplied throughbrushes 19 and the slip rings 18 with current from a control circuit forthe electromagnetic powder clutch.

When the magnetizing coil 15 is excited by the clutch current, thedriven member 14 is magnetized to produce a magnetic flux passingthrough the drive member 12. The magnetic powder is aggregated in thegap 16 by the magnetic flux and the driven member 14 is engaged with thedrive member 12 by the powder. On the other hand, when the clutchcurrent is cut off, the drive and driven members 12 and 14 aredisengaged from one another.

The selector device 2 is provided between the input shaft 13 and a mainshaft 20. The main shaft 20 is cylindrical and is disposed coaxiallywith the input shaft 13. The selector device 2 comprises a drive gear 21integral with the input shaft 13, reverse driven gear 22 rotatablymounted on the main shaft 20, and a synchronizer 27 mounted on the mainshaft 20. The drive gear 21 meshes with one counter gear 24 rotatablymounted on a shaft 23. Another of the counter gears 24 engages with anidler gear 26 rotatably mounted on a shaft 25, which in turn engageswith the driven gear 22.

The synchronizer 27 comprises a hub 28 secured to the main shaft 20, asynchronizer sleeve 29 slidably engaged with the hub 28 with splines,and synchronizer rings 30 and 31. The synchronizer sleeve 29 is adaptedto engage with splines of the drive gear 21 or with splines of thedriven gear 22 through the rings 30 or 31.

At a neutral position (N range) or a parking position (P range) of aselector lever (not shown), the sleeve 29 does not engage either gear,so that the main shaft 20 is disconnected from the input shaft 13. Whenthe sleeve 29 is engaged with the gear 21, the input shaft 13 isconnected to the main shaft 20 through the gear 21 and synchronizer 27to provide a drive range position (D range). When the sleeve 29 isengaged with the gear 22, the input shaft 13 is connected to the mainshaft 20 through the gears 21, 24, 26 and 22 to provide a reverse driverange position (R range).

The main shaft 20 has an axial passage in which an oil pump drivingshaft 42 connected to crankshaft 10 is mounted. In the transmission 3,an output shaft 35 is provided in parallel with the main shaft 20. Adrive pulley 36 and a driven pulley 37 are mounted on shafts 20 and 35.A fixed conical disc 36a of the drive pulley 36 is integral with themain shaft 20 and an axially movable conical disc 36b is axiallyslidably mounted on the main shaft 20. The movable conical disc 36b alsoslides in a cylinder 38a secured to the main shaft 20 to form a servodevice 38 having a chamber 38b.

A fixed conical disc 37a of the driven pulley 37 is formed on the outputshaft 35 opposite the movable disc 36b and a movable conical disc 37b isslidably mounted on the shaft 35 opposite disc 36a. The movable conicaldisc 37b also slides in a cylinder 39a secured to the output shaft 35 toform a servo device 39. A chamber 39b of the servo device 39 iscommunicated with the oil pump 41 through a passage 72. A spring 40 isprovided to urge the movable conical disc 37b to the fixed conical disc37a. A drive belt 34 engages with the drive pulley 36 and the drivenpulley 37. The movable disc 36b has an area exposed to the line pressurewhich is larger than the corresponding area of the movable disc 37b.

Secured to the output shaft 35 is a drive gear 43 which engages with anintermediate reduction gear 44a on an intermediate shaft 44. Anintermediate gear 45 on the shaft 44 engages with a final gear 46.Rotation of the final gear 46 is transmitted to axles 48 and 49 of frontdriving wheels of the vehicle (not shown) through a differential 47.

The transfer device 5 comprises a transfer gear 50 engaged with thefinal gear 46 and rotatably mounted on a transfer shaft 51 which istransversely disposed and rotatably supported by bearings and a transferclutch 52 in the form of a fluid operated multiple-disk friction clutch.The transfer clutch 52 comprises a hub 56 secured to the transfer gear50, cylinder 57 secured to the shaft 51, a plurality of disks 59provided between the hub 56 and the cylinder 57, and piston 58 forengaging the disks 59 with each other and piston chamber 61. A spring 60is provided to urge the piston 58 to disengage the disks 59. Thus, thetransfer clutch 52 operates to couple the gear 50 with the shaft 51, sothat the rotation of the gear 50 is transferred to rear drive wheels ofthe vehicle (not shown) through shaft 51, bevel gear 53 secured to theshaft, bevel gear 54 engaged with the bevel gear 53 and rear drive shaft55. The piston chamber 61 communicated through a passage 65 in the shaft51 with a value assembly 63 which is provided with a solenoid 64.

FIGS. 2a to 2c show a hydraulic circuit control according to the presentinvention. The circuit is provided with a pressure regulator valve 80and a transmission ratio control valve 90.

The pressure regulator valve 80 comprises a valve body 81, spool 82, aspring 84 provided between a spring retainer 83 and one end of the spool82 for urging the spool to the left.

The transmission ratio control valve 90 comprises a valve body 91, spool92, operating plunger 93, and a spring 94 provided between the spool 92and the plunger 93. Oil in an oil reservoir 70 is supplied to a port 81bof the pressure regulator valve 80 through a passage 71 by the pump 41.An oil passage 72 connected to the passage 71 is communicated with thedriven pulley servo chamber 39b. The passage 71 is further communicatedwith the drive pulley servo chamber 38b through ports 81c and 81g of theregulator valve 70, the transmission ratio control valve 90 and apassage 73. Oil flows back from a port 81d of the regulator valve 80 tothe inlet of the pump 41 through drain passages 74 and a check valve 78.A drain port 91d of the transmission ratio control valve 90 iscommunicated with the oil reservoir 70 through a passage 75 and a checkvalve 78a.

The drive pulley cylinder 38a has an annular inside groove 38c in whicha rotation speed sensor 76 in the form of a pitot tube is provided formeasuring the speed of the oil in the groove, that is the speed of themain shaft 20 which is proportional to the engine speed. The pitotpressure produced by the rotation speed sensor 76 is applied to endchambers 81a and 91a of the valves 80 and 90 through passages 77 and77a.

The drain passage 74 for the pressure regulator valve 80 is communicatedwith a select position detecting valve 110 upstream of the check valve78 through a passage 79 and further with an actuator 120 for thetransmission ratio control valve 90 through a passage 88.

A sensor shoe 85 for detecting the actual transmission ratio is slidablymounted on a lubricating oil pipe 86 which is parallel with the axis ofthe spool 82. A bolt 85a secured to an end of the sensor shoe 85 engageswith an end of a bolt 83a secured to the spring retainer 83, and theother end of the sensor shoe 85 engages with the outside periphery 36dof the movable disc 36b. Thus, the position of the movable disc 36bwhich means the transmission ratio during the operation is transmittedto the spool 82 through the spring 84. At the end of the valve body 81,opposite to the spring 84, the pitot pressure is applied to the endchamber 81a, and pump oil pressure is applied to chamber 81b through thepassage 71. A chamber 81e which is provided between the chambers 81a and81b for preventing the leakage of the oil dependent on the pitotpressure, is communicated with the oil reservoir 70 through drainpassages 74a. The port 81c is communicated with the port 81d through achamber formed on a land 82a of the spool 82 when the spool is shiftedto the right, so that the line pressure can be regulated.

Thus, the spool 82 is applied with the pitot pressure and pump oilpressure so as to be moved in the direction to open the port 81d,whereas the elastic force of the spring corresponding to thetransmission ratio detected by the sensor shoe 85 urges the spool 82 inthe direction to close the port 81d. Accordingly, high line pressure isgenerated at the port 81g at a low engine speed with a largetransmission ratio. The sensor shoe 85 is moved to the right in FIG. 2bas the transmission ratio decreases, reducing the force of the spring 84to lower the line pressure. The line pressure is transmitted to the belt34 at a proper force dependent on the transmission ratio so as not tocause the belt to slip on the pulleys.

The transmission ratio control valve 90 further comprises an annulargroove 92a formed on the spool 92 so as to communicate the ports 91bwith 91c or port 91d for supplying or discharging line pressure to orfrom the drive pulley servo chamber 38b in dependency of the position ofthe spool 92. A regulator spring 97 is provided between the operatingplunger 93 and a retainer 96 securely mounted on a projecting end of aregulator plunger 95 which is slidably provided in an axial cavity 92cin the spool 92. A spring 98 is provided between a flange of the plunger95 and a retainer 92b of the spool 92. The force of the regulator spring97 is determined by the projecting extent of the plunger 95 from thespool 92 and the position of the plunger 95 is dependent on the linepressure at the port 91c which is supplied to the inside of the spool 92through a small aperture 99.

The plunger 93 is slidably mounted in the valve body 91 and has an axialcavity 93c. A rod 101 is axially slidably mounted in the valve body 91,and a flange 103 of the rod 101 is slidably engaged with the wall of thecavity 93c. A small spring 102 is provided between the flange 103 andthe plunger 93. The pitot pressure is applied to the chamber 91a througha port 92d. The cavity 93c is supplied with the pitot pressure through aport 91f and a pasasge 106 having an orifice 105. A spring 107 isprovided between an end of the spool 92 and the valve body 91 to adjustthe load on the spring 102. An end of the rod 101 engages wtih a cam 100which is operatively connected to an accelerator pedal 100a of thevehicle so as to be rotted in dependency on the depression of the pedal.

When the spool 92 is moved by the pitot pressure to communicate the port91c with port 91b, the line pressure is applied to the servo chamber 38bof the drive pulley 36 to upshift the transmission. On the other hand,when the port 91b communicates with the port 91d, the chamber 38b isdrained to downshift the transmission.

The select position detecting valve 110 comprises a valve body 111, avalve 113 having a drain aperture 112 which is slidably mounted in thevalve body 111, a spring 114 for urging the valve 113 against a cam 115which rotates according to the postion of a selector lever 117. The cam115 has a lobe 115a corresponding to D, N, R range positions, and lobes115b formed in the both sides of the lobe 115a, corresponding to P andDs range positions. At the D, N, R range positions, the lobe 115a pushesthe valve 113 in the direction to close the drain aperture 112, so thatactuating oil pressure is built up. At the P and Ds range positions, thevalve 113 moves outwardly to open the drain aperture 112, so that theoil pressure in the passages 74 and 79 is lowered. At that time the oilpressure decreases gradually, because of an orifice 116 provided in theoil passage 79. The oil pressure is applied to the actuator 120.

The actuator 120 comprises a cylinder 121, a piston 122 which isslidably mounted in the cylinder 121, and a spring 123 for urging thepiston 122 in a direction toward a piston chamber 124 to which actuatingoil pressure is applied through a passage 88. Further, a connecting rod125 is secured to an outer end of a rod 122a which is connected to thepiston 122 and is engageable with a pin 126 on the rod 101 of thetransmission ratio control valve 90. At the P range or Ds range, thesince no acting oil pressure exists, piston 122 presses the rod 101 tothe left in FIG. 2c, controlling the transmission zone to the side ofhigh engine revolution. Thus, the releasing of the accelerator pedal atDs range causes the downshifting of the transmission, so that thirdengine braking effects. Further, a correction lever 128 rotatablysupported by a pin 127 is provided between the sensor shoe 85 and therod 125, in order to correct the charcteristics in the Ds range. One endof the lever 128 engages the with the rod 125 only when piston 122 ofthe actuator 120 moves to the left. Under such a condition, when thetransmission is downshifted to shift the disk 36b to the left, thesensor shoe 85 engages with the end of the lever 128 to rotate it. Thus,the rod 125 and piston 122 are moved toward the right as thetransmission ratio increases, and at the maximum transmission ratio, thepiston 122 is returned to the right end position.

A control system for the transfer clutch 52 is explained hereinafter.The control system comprises a pressure regulator valve 140, a transfercontrol valve 150, a solenoid operated valve 133, and a control unit160. The pressure regulator valve 140 comprises a valve body 141, spool142, end chamber 143, spring 145 for urging the spool to the end chamber143. A port 141a is commuinated with the pump 41 through a passage 130having an orifice 135. The spool 142 is shifted by the differencebetween forces exerted on both ends thereof, one of which is thepressure of oil in the chamber 143 and the other is the force exerted bythe spring 145. When the spool 142 is downwardly shifted, a drain port141b opens, thereby draining the oil in the chamber 143 through apassage 144 and ports 141c and 141b. When the spool 142 is upwardlyshifted, the port 141a opens, so that oil is supplied to the chamber 143through passage 130, ports 141a and 141c, and passage 144. Thus, thepressure of oil in the chamber 143 is kept constant.

Designating the pressure receiving area of the spool 142 in the chamber143 by S, regulator pressure of oil in the chamber 143 by Pr and theforce exerted by spring 145 by F,

    Pr·S=F

    Pr=F/S

Accordingly, the pressure Pr can be kept constant.

The transfer control valve 150 comprises a valve body 151, a spool 152having two pistons which are different in diameter, an end chamber 153communicated with the chamber 143 through a passage 153a having anorifice 153b, a spring 154, a port 151a communicated with the passage130 through a passage 131, a port 151b communiated with the pistonchamber 61 of the transfer clutch 52 through passage 65, and drain port151c. The chamber 153 is communicated with the solenoid operated valve133 through a passage 132 so as to provide control pressure in chamber153. The control pressure Pc and clutch pressure P_(T) of oil in thechamber 61 urge the spool 152 downwardly against the spring 154. Whenthe spool moves downwardly, port 151a is closed and drain port 151c isopened, so that the clutch pressure P_(T) decreases. To the contrary,when the spool 152 is upwardly shifted, port 151a opens to increase theclutch pressure P_(T).

Thus equation for the control pressure Pc, clutch pressure P_(T), theforce F exerted by the spring 154, the area S1 of the large diameterpiston and the area S2 of the small diameter piston of the spool 152, is

    Pc·S2+P.sub.T (S1-S2)=F

Therefore

    P.sub.T =(F-Pc·S2)/(S1-S2)

Since S1, S2 and F are constant, the clutch pressure P_(T) is inverselyproportional to the control pressure P_(c). The solenoid operated valve133 is operated by a pulse train applied from the control unit 160. Thecontrol pressure Pc is controlled by changing the duty ratio of thepulses in the pulse train.

FIG. 4a shows the relationship between the clutch pressure P_(T),control pressure Pc and duty ratio. When the duty ratio is at 0%, thesolenoid operated valve 133 does not drain through the drain passage133a. Accordingly, the control pressure is at the highest value which isequal to the regulator pressure Pr. As the duty ratio increases, thecontrol pressure Pc reduces as shown by the dashed line. On the otherhand, the clutch pressure P_(T) is zero in a range smaller than a smallduty ratio d1 and increasing with increase of the duty ratio from theduty ratio d1.

Referring to FIGS. 3a to 3c showing a block diagram of a systemaccording to the present invention, the system has a 4WD switch 161 fordetecting the four-wheel drive state, accelerator switch 162 for thedetection of the release of the accelerator pedal 100a, throttleposition (switch) 163 for detecting whether the engine load is in alight load range or a heavy load range, engine speed sensor 164, vehiclespeed sensor 165 for detecting vehicle speed dependent on the speed ofthe front wheels, shift range switch 166 for detecting D, Ds and Rranges, brake switch 167 for sensing the depression of a brake pedal,clutch current sensor 168 of the clutch 1, and oil temperature sensor169. When the 4WD switch 161 is closed, output signals of the switchesand sensors 162-169 are fed to the control unit 160 through an interface170.

In the control unit 160, a throttle position and transmission ratiocalculator 171 calculates the transmission ratio based on engine speedand vehicle speed and the position of a throttle valve of the engine istaken from a look-up table. The relationship between vehicle speed andengine speed of a vehicle having the infinitely variable belt-drivetransmission has a pattern in accordance with load as shown in FIG. 4c.Accordingly a look-up table for the throttle valve position can beprovided from the graph of FIG. 4c. In a low vehicle speed range D₁lower than a low speed V₁, the clutch 1 is in a partial engage state inaccordance with small clutch current shown in FIG. 4b. A clutch torquecalculator 172 calculates the torque capacity, that is the transmittedtorque in the partial engage state of the clutch 1, based on the clutchcurrent. An engine torque look-up table 173 provides engine torque fromthe throttle position and engine speed. An oil temperature detector 174detects high temperature higher than a set value.

A driving condition detecting section 175 comprises a low vehicle speeddetector 175a for detecting vehicle speed lower than the speed V₁,clutch entire engage detector 175b at high speed higher than V₁, andbrake operation detector 175c. The output signal of the low vehiclespeed detector 175a is applied to a load range detector 176 whichdetects whether the engine is in a light load range or a heavy loadrange in accordance with output signals of the accelerator switch 162and throttle position sensor 163, and further detects the position ofthe shift lever 117 at each load range in accordance with the outputsignal of the shift range switch 166. The output signal of the detector175b is applied to a load range detector 177 which detects a light loadrange D₂ (FIG. 4c) below a predetermined engine speed N₁ and a heavyload range D₃ above speed N₁. The engine speed N₁ is set to a speedlower than an engine speed N₂ from which the upshifting of thetransmission starts. The output signal of the brake operation detector175c is applied to a deceleration calculator 178 for producing adeceleration signal. The output signal of each section is applied to atransfer clutch torque capacity calculator 180. A clutch torque rate(percentage) memory 179 supplies torque rates to the torque capacitycalculator 180 so as to provide a proper torque capacity.

In a low vehicle speed range below a set speed V1 in partial engagementof the clutch 1, the torque capacity of the transfer clutch 52 isdetermined by a predetermined percentage of the transmitting torque ofthe clutch 1. When the clutch 1 is entirely engaged above the speed V1,the torque capacity is determined by other percentages with respect tothe engine torque.

A duty ratio setting section 181 produces a duty ratio signal dependenton the output of the torque capacity calculator 180. The duty ratiosignal is corrected by a coefficient from a coefficient memory 183 whenoil temperature is higher than a set temperature t₁. The duty ratiosignal is applied to the solenoid operated valve 133 through a driver182.

One of the outputs of the transfer clutch torque capacity calculator 180is connected to a clutch pressure (torque capacity) reducing signaldetector 185. The calculator 180 applies a signal to the clutch pressurereducing signal detector 185, time whereby the calculator supplies aclutch pressure reducing signal to the duty ratio setting section 181through the section 184, for example at opening the 4WD switch 161 orchanging driving ranges. The output of the detector 185 is applied to aclutch pressure reduction rate setting section 184 the output of whichis applied to the duty ratio setting section 181. In response to theoutput of the section 184, the section 181 produces an output forgradually decreasing the duty ratio.

The operation of the system is described hereinafter. While the vehicleis at a stop, the driven pulley servo chamber 39b is supplied with theline pressure adjusted by the pressure regulator valve 80 through thepassages 71, 72, and the drive pulley servo chamber 38b is drained,since the spool 92 is at the left end position by the spring 94. Thus,the driving belt 34 engages with the driven pulley 37 at a maximumrunning diameter to provide the largest transmission ratio (low speedstage).

On the other hand, the selector lever 117 is at the N range position toselect the N range, so that the drain aperture 112 of the selectposition detecting valve 110 is closed. When the D range is selected,the input shaft 13 and the main shaft 20 are connected to each other inthe selector device 2 by the selector lever 117. When the accelerationpedal is depressed, the electromagnetic powder clutch 1 is excited by aclutch current, transmitting the engine power to the drive pulley 36.The power of the engine is transmitted to the output shaft 35 at thelargest transmission ratio by the driving belt 34 and driven pulley 37,and further transmitted to axles of the driving wheels. Thus, thevehicle is started.

When the pitot pressure rises, the spool 92 is shifted to the left, sothat the line pressure is applied to the servo chamber 38b to move thedisc 36b to the left. Thus, upshifting of the transmission starts.

When the 4WD switch 161 is off, the control unit 160 produce a two-wheeldrive signal, so that the duty ratio setting section 181 produces a dutyratio signal for providing pulses of zero percent. Accordingly, thesolenoid operated valve 133 is closed, thereby increasing the controlpressure P_(c) in the chamber 153 of the transfer control valve 150 to amaximum value. As a result, the spool 152 is downwardly shifted to closethe port 151a, which causes the clutch pressure P_(T) to become zero todisengage the transfer clutch 52. Thus, two-wheel drive transmission isestablished.

When the 4WD switch 161 is closed, signals from switches and sensors areapplied to the control unit 160 through the interface 170. At a lowvehicle speed below V₁, including the starting of the vehicle, thetorque capacity of the electromagnetic clutch 1 is calculated at thecalculator 172. Thus, the partial engagement of clutch 1 is detected bythe value of the torque capacity. If the brake switch 167 is off, theload range detector 176 determines load range of the engine. When theaccelerator pedal is released and the accelerator switch 162 is off, thetorque capacity calculator 180 produces an output signal representing aminimum duty ratio (a minimum percentage (10%) of engine torque at fullthrottle). Accordingly, torque capacity of the transfer clutch 52 isfixed to ten percent of the engine torque at full throttle. Thus, asmall torque is transmitted to the rear wheels of the vehicle at a lowvehicle speed, so that the tight braking phenomenon can be preventedwith slipping of the transfer clutch.

When the accelerator switch 162 is closed, and the throttle positionswitch 163 is off because of a light load on the engine, the torquecapacity calculator 180 produces a signal for a small percent of theclutch torque transmitted by the clutch 1. Accordingly, the torquecapacity of the transfer clutch increases. Further, the torque capacityof the transfer clutch increases with an increase of the torquetransmitted by the clutch 1. In such a state, the transfer clutch stillslips so as to prevent the tight corner braking.

When the throttle position switch 163 is on at a heavy load, thecalculator 180 produces a signal for a large percent. Thus, the torquecapacity of the transfer clutch 52 increases.

When the vehicle speed is higher than the speed V₁ while the clutch 1 isentirely engaged, the engine torque look-up table 173 provides enginetorques dependent on vehicle speeds, for example as shown by lines L₁,L₂, and L₃ in FIG. 6. In response to the output signal of clutch entireengage detector 175b, the range detector 177 detects whether the drivecondition is in the range D₂ or D₃.

In the range D₂, the calculator 180 produces a signal for thirty percentof engine torque (L₁, L₂, L₃). Accordingly, the torque capacity of thetransfer clutch 52 is set as shown by lines L₁ ', L₂ ' and L₃ '. Thus,thirty percent of the engine torque is transmitted to the rear wheels toallow the transfer clutch to slip at sharp curves.

In the range D₃, the torque rate is set to fifty percent. In such acase, the engine torque at full throttle is used for obtaining thetorque capacity of the transfer clutch. FIGS. 5a to 5c show theabove-described operation.

As shown in FIG. 6, when the engine torque decreases, the torquecapacity of the transfer clutch decreases. For example, in the case thatthe torque capacity of the transfer clutch 52 reduces from line l₁ ' tol₂ ' at a time T₁ (FIG. 7), the reduction is detected by the clutchpressure reducing signal detector 185 and the clutch pressure reductionrate setting section 184 produces an output signal. Thus, the duty ratiosetting section 181 operates to gradually reduce the duty ratio. Thus,the control pressure P_(c) is gradualy increased, so that clutchpressure P_(T) and thus the clutch torque is gradually reduced as shownby a line m₁ in FIG. 7.

When the accelerator pedal is depressed at a time T₂, the clutchpressure P_(T) increases from a point M as shown by a line m₂. Thus, thetorque capacity rapidly increases to a set value dependent on thedriving conditions. In the conventional system with the torque capacitychange from l_(i) to l₂ l at time T_(i) and acceleration pedal depresionat time T₂. as described above, the clutch pressure varies as shown by adashed lines m₃ and m₄. Accordingly, the clutch pressure suddenlyreduces and a longer time elapses before the clutch pressure reaches theset value.

While the presently preferred embodiment of the present invention hasbeen shown and described, it is to be understood that this disclosure isfor the purpose of illustration and that various changes andmodifications may be made without departing from the scope of theinvention as set forth in the appended claims.

What is claimed is:
 1. A control system for a four-wheel drive vehiclehaving an engine, a transmission, means comprising a main clutch fortransmitting the output of the engine to the transmission, main drivewheels operatively connected to an output shaft of the transmission,auxiliary drive wheels, means comprising a transfer clutch fortransmitting output of the transmission to the auxiliary drive wheels,the transfer clutch having a controllable clutch torque capacity, thesystem comprising:a vehicle speed sensor provided to detect vehiclespeed and to produce a vehicle speed signal; means for providing anengine torque signal representative of engine torque dependent on anoperating condition of the engine; first control means responsive to thevehicle speed signal and the engine torque signal for engaging thetransfer clutch at a torque capacity which is determined by apredetermined rate of the engine torque in accordance with the vehiclespeed signal; detector means responsive to a transfer clutch torquecapacity reducing signal for producing a clutch torque reducing signal;second control means responsive to the clutch torque reducing signal forgradually reducing the torque capacity determined by the first controlmeans to a predetermined value.
 2. The control system according to claim1 further comprisingmeans for producing oil pressure, the transferclutch is a fluid operated friction clutch operated by the pressure ofthe oil, and said second control means gradually reduces the pressure ofthe oil in response to the clutch torque reducing signal for reducingthe torque capacity.
 3. In a control system for a four-wheel drivevehicle having an engine, a variably engageable main clutch connectedfor transmitting output of the engine to a transmission, main drivewheels operatively connected to an output shaft of the transmission,auxiliary drive wheels, and a variably engageable transfer clutchconnected for transmitting the output of the transmission to theauxiliary drive wheels, the improvement comprisinga vehicle speed sensorfor producing a vehicle speed signal dependent on vehicle speed, athrottle position sensor responsive to engine load in a light load rangeand a heavy load range, respectively, for producing a throttle positionsignal depending on the ranges, respectivly, means for determiningtorque capacity of the main clutch for producing a main clutch torquesignal dependent on the torque capacity of the main clutch, controlmeans responsive to the vehicle speed signal, said main clutch torquesignal and the throttle position signal for controlling torque capacityof the transfer clutch, in accordance with vehicle speed, topredetermined rates of said main clutch torque capacity in dependency onthe throttle position signals respectively.
 4. The control systemaccording to claim 3, whereinsaid control means further comprises meansfor gradually reducing the transfer clutch torque capacity in responseto an operating condition to prevent shock.
 5. The control systemaccording to claim 4, whereinsaid operating condition is opening of 4WDswitch operatively connected to the control means.
 6. A control systemfor a four-wheel drive vehicle having an engine, a transmission, meanscomprising a main clutch for transmitting the output of the engine tothe transmission, main drive wheels operatively connected to an outputshaft of the transmission auxiliary drive wheels, and means comprising atransfer clutch for transmitting output of the transmission to theauxiliary drive wheels, the transfer clutch having a controllabletransfer clutch torque capacity, comprising:a vehicle speed sensorprovided to detect vehicle speed and to produce a vehicle speed signaldependent on the vehicle speed; means for providing an engine torquesignal representative of engine torque capacity and dependent on anoperating condition of the engine; and control means responsive to thevehicle speed signal and the engine torque signal for engaging thetransfer clutch at a transfer clutch torque capacity which is determinedto be a predetermined rate of the engine torque capacity in accordancewith the vehicle speed signal.
 7. The control system according to claim6, whereinsaid control means is for engaging the transfer clutch at atransfer clutch torque capacity which is determined to be apredetermined rate of the engine torque capacity at full throttle undercertain vehicle conditions respectively.
 8. The control system accordingto claim 6, whereinsaid vehicle conditions are the vehicle speed beinggreater than a predetermined value, the main clutch being fully engagedand the transmission being in a heavy load range, and respectively, thevehicle speed being lower than the predetermined value, an acceleratorpedal of the vehicle being released, and the main clutch being partiallyengaged.
 9. A control system for a four-wheel drive vehicle having anengine, a transmission, means comprising a variably engageable mainclutch for transmitting the output of the engine to the transmission,main drive wheels operatively connected to an output shaft of thetransmission, auxiliary drive wheels, means comprising a transfer clutchfor transmitting output of the transmission to the auxiliary drivewheels, and the transfer clutch having a controllable transfer clutchtorque capacity, comprising:a vehicle speed sensor provided to detectvehicle speed and to produce a vehicle speed signal; means fordetermining torque capacity of the main clutch for producing a mainclutch torque signal dependent on the torque capacity of the mainclutch; and first control means responsive to the vehicle speed signaland the main clutch torque signal for engaging the transfer clutch at atransfer clutch torque capacity which is determined to be apredetermined rate of the main clutch torque capacity in accordance withthe vehicle speed signal.
 10. The control system according to claim 7,further comprisingdetector means responsive to a transfer clutch torquecapacity reducing signal for producing a transfer clutch torque reducingsignal; and second control means responsive to the clutch torquereducing signal for gradually reducing the transfer clutch torquecapacity determined by the first control means to a predetermined value.